Variable optical gain control device

ABSTRACT

There is provided a variable optical gain control device capable of amplifying the luminous intensity of an optical signal while the luminous intensity of the optical signal is controlled. The device includes an optical circuit  100  which is formed on a flat face of a substrate and is provided with an input port  121  for receiving an optical input signal  122  inputted thereto; an input port  123  for receiving a pump laser light  124  inputted thereto; the first directional coupler  125  for joining up the input optical signal  122  and the pump laser light  124 ; a luminous intensity control portion  106  for controlling the luminous intensity of the optical signal combined with the pump laser light  124 : the second directional coupler portion  126  for separating the pump laser light  124  from the optical signal  122  once combined therewith; an output port  127  for outputting the controlled optical signal; and an output port  129  for outputting the pump laser light. The optical waveguide path portion  110  is doped with a rare earth element and the optical signal is amplified by the function of the pump laser light, so that it becomes possible to amplify the optical signal while the luminous intensity of the optical signal is controlled.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical circuit of a variable optical gain control device, which is used for controlling the luminous intensity in an optical communication system, and also to the structure of the optical circuit.

[0003] 2. Description of the Related Art

[0004] Nowadays, a lot of the optical communication systems in the world introduce a Wavelength Division Multiplexing (WDM) system as the most effective systems for increasing the optical transmission capacity. Heretofore, in the WDM system, as a means for arbitrarily controlling the luminous intensity to an arbitrary value, there has been used a Variable Optical Attenuator (VOA) having the function of changing the attenuation quantity of the luminous intensity. In view of the increase tendency of such an optical communication system provided with the WDM system, there is being still vigorously carried out the technological development on such a VOA that is able to change the attenuation quantity of the luminous intensity in response to an electrical signal.

[0005] In the WDM system, the VOA is used for two principal purposes. One is to equalize the level of the optical signal which is different on every wavelength thereof, and the other is to keep the output level of the optical signal constant even if there are changed the wavelength and the input level of the optical signal to be amplified when the optical amplification factor is made constant in a multi-wavelength collective optical amplifier. The reason for keeping the optical amplification factor constant is for preventing the deterioration of the system characteristic caused by variation in the wavelength characteristic of the optical amplifier

[0006] As one of prior art VOA's, there is a VOA of the non-mechanical type which makes use of the thermo-optic effect. Typically, the VOA of this type is provided with a Mach-Zehnder interferometer formed on the way of a glass-made optical waveguide path. Each of two optical paths of this interferometer is provided with a heater, and the phase difference of the optical signal in each waveguide path between these optical paths is changed by the temperature difference between two heaters. The VOA of this type can be arranged in the form of an array with ease, and the VOA having 20 to 40 channels is now being produced and on the market already.

[0007] In case of executing the optical gain control by means of the prior art VOA, however, as the luminous intensity is controlled by attenuating it, there happens such inconvenience the luminous intensity can not help being smaller than the original one.

[0008] On one hand, if it is tried to control the optical gain by mean of an Erbium Doped Fiber Amplifier (EDFA), problems to be considered would come up, for instance, large noise generation, increase in the dimension of the entire system because the optical fiber has to be wound many times before putting it in a predetermined package case, and so forth. Furthermore, in case of controlling the optical gain by mean of a semiconductor optical amplifier, the coupling loss between the amplifier and the optical fiber becomes so large that dependency on biased wave is made stronger.

SUMMARY OF THE INVENTION

[0009] Accordingly, the invention has been made in view of the problems as described above, and an object of the invention is to provide a novel and improved variable optical gain control device which causes not only less noise but also less coupling loss, and makes it possible to amplify the luminous intensity.

[0010] In order to solve the problems as described above, according to the invention, there is provided a variable optical gain control device having an optical circuit constituted by means of optical waveguide paths formed on a flat face of the substrate, the optical circuit including the first port for receiving an optical signal inputted thereto; the second port for receiving an excitation light inputted thereto; the first directional coupler portion for joining up the optical signal and the excitation light; a luminous intensity control portion, for instance made up of a circuit of the Mach-Zehnder type, for controlling the luminous intensity of the optical signal combined with the excitation light: the second directional coupler portion for separating the excitation light from the optical signal of which the luminous intensity has been controlled by the luminous intensity control portion; the third port for outputting the luminous controlled optical signal; and the fourth port for outputting the excitation light as separated. The optical waveguide path is doped with a rare earth element, and the optical signal is amplified by the function of the excitation light and is then outputted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will now be described in detail by way of an example of a variable optical gain control device embodying the invention with reference to the accompanying drawings. In the following description and drawings, elements having a substantially identical function and constitution of the invention are designated by an identical reference numeral and character in order to avoid iterative and redundant descriptions. In the drawings:

[0012]FIGS. 1A and 1B are diagrams showing a variable optical gain control device according to the first embodiment of the invention, wherein FIG. A is a plan view of the variable optical gain control device and FIG. 1B is a sectional view taken on line A-A of FIG. 1A,

[0013]FIG. 2 is a plan view showing a variable optical gain control device according to the second embodiment of the invention, and

[0014]FIG. 3 is a plan view showing a variable optical gain control device according to the third embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] (First Embodiment)

[0016] Referring to FIG S. 1(a) and 1(b), the former is a plan view showing the optical circuit of the fist embodiment of the invention while the latter is a sectional view of the above optical circuit taken on line A-A of FIG. 1A. In the first embodiment of the invention, an optical circuit of the Mach-Zehnder type is used as a luminous intensity control portion.

[0017] The Mach-Zehnder type optical circuit has such a structure that an optical waveguide path is once divided into two arms (optical waveguide paths), thereby forming a branch arm portion, and then, these two arms are again joined up with each other. One arm of this branch arm portion is provided with a thin film heater. With the thermo-optic effect given by the thin film heater, the Mach-Zehnder type optical circuit is able to change the refractive index of the optical waveguide path and the optical interference state as well. With this, the Mach-Zehnder type optical circuit makes it possible to control the luminous intensity.

[0018] As shown in FIG. 1A, the optical circuit 100 is divided into three portions, that is, the first WDM coupler portion 101, a Mach-Zehnder portion 102, and the second WDM coupler portion 103. In the Mach-Zehnder portion 102, one arm 111 is provided with a heater 113, for instance a chromium-made heater. As shown in FIG. 1B, the sectional structure of a substrate, on which the optical waveguide path is formed, is constituted with a silicon substrate 10, a lower clad layer 11 with a film thickness of about 20 μm, cores 12 and 13 each of which forms an optical waveguide path 110 having a film thickness of about 5 μm and a width of about 5 μm, an upper clad layer 14 covering the cores 12 and 13 and having a film of about 15 μm thickness, and a heater 113 located above one core 12. Both of the lower and upper clad layers 11 and 14 are made of silicon dioxide.

[0019] The main component forming cores 12 and 13 is silicon dioxide including a certain amount of a doped impurity, for instance erbium (Er) of about 2 wt %. If an optical signal is inputted to the optical waveguide path together with the excitation light, the optical signal can be amplified with the inductive amplification function of the Er ion. To put it concretely a little, if the excitation light (pump laser light) is inputted to the Er-doped optical waveguide path, the optical waveguide path absorbs the light having a wavelength of 0.98 μm and 1.48 μm among from the excitation light as inputted and, then begins to emit the light having the wavelength of 1.53 μm or so. The optical signal is given luminous energy and can be amplified by making use of this inductive discharge phenomenon in association with this light emission.

[0020] The excitation efficiency varies depending on the amount of a dopant such as Er in this case, and the gain of the luminous amplification is determined by the excitation efficiency. It should be noted, however, that the amount of the dopant as doped is to be within a certain restricted range. If the dopant is doped to exceed or to fail to reach that range, it would bring about an adverse effect or nothing. In this first embodiment, a preferable doping ratio of the dopant is 0.2 through 3.0 wt %.

[0021] In the first WDM portion 101, an input port 121 as the first port is connected with an optical fiber to which an input optical signal 122 having a wavelength band of 1.55 μm is inputted. From an input port 123 as the second port, there is inputted a pump laser light 124 having a wavelength of 1.48 μm or 0.98 μm. The input optical signal 122 inputted from the input port 121 is combined with the pump laser light 124 by means of a directional coupler 125 as the first directional coupling portion and propagates through an Er-doped optical waveguide path 110 to input to a Mach-Zehnder portion 102 as a luminous intensity control portion.

[0022] The input optical signal 122 inputted to Mach-Zehnder portion 102 is divided into two optical signals by means of the directional coupler 104 located on the left side of the Mach-Zehnder portion 102. These two divided optical signals are given to two arms 111 and 112 respectively such that the ratio of their luminous intensities becomes 1:1. While no electric current is given to a heater 113 of the arm 111, the divided input optical signals join up in phase through the directional coupler 105, thus the divided input optical signals joining up and propagating without causing any luminous emission.

[0023] If an electric current is applied between the heater terminals 131 and 132 of the arm 111, the heater 113 begins to change its temperature. As a result, the refractive index of the optical waveguide path located immediately below the heater 113 is changed due to the thermo-optic effect, thus the phase of the optical signal propagating through the arm 111 being change and then, the phase-changed wave is combined with the phase-unchanged wave by the directional coupler 105 located on the right side of the Mach-Zehnder portion 102. At this time, a luminous power is emitted toward the optical waveguide path substrate and attenuates in correspondence with the phase difference between the above two optical signals. Accordingly, the maximum attenuation takes place when two optical signals are in reverse phase. In this way, the luminous intensity controlled light is outputted in response to the phase difference between two light waves.

[0024] The optical signal controlled by the Mach-Zehnder portion 102 is propagated to the second WDM coupler potion 103, where the pump laser light 124 is separated by the directional coupler 126 as the second directional coupler portion. Then, an output optical signal 128 is put out from the output port 127 as the third port.

[0025] At this time, however, the optical signal intensity of the output optical signal 128 is amplified by about 10bB due to effects of the Er-doped optical waveguide path 110 and the pump laser light 124. The separated pump laser light having a wavelength of 1.48 μm or 0.98 μm is outputted from the output port 129 as the fourth port, but not from the output port 127.

[0026] Ideally, the pump laser light 124 incident on the first WDM coupler portion 101 can amplify the optical signal of the Er-doped optical waveguide path at an energy conversion rate of 100%. Actually, however, such a high energy conversion rate can not be obtained because of the light reflection at the end face of the optical waveguide path, the light leakage and so force, which are caused by the characteristic dispersion of the first WDM coupler portion 101 induced in the course of manufacturing it. Accordingly, the WDM coupler portion has to be designed by sufficiently taking account of such dispersion. Unless erbium is doped to the optical waveguide path 110, any amplification would not take place even if the excitation pulse laser light is incident on the first WDM coupler portion 101.

[0027] As has been discussed above, according to the first embodiment of the invention, there is provided a variable optical gain control device wherein there are formed on the same flat face of the substrate a luminous intensity control portion using a circuit of the Mach-Zehnder type for instance, and an optical amplification mechanism amplifying the luminous intensity by using the optical waveguide path doped with a rare earth element as a dopant. With this, it becomes possible to amplify the optical signal without causing any attenuation of the optical signal while the luminous intensity of the optical signal is controlled. Accordingly, in the WDM system, it becomes possible to omit an optical fiber amplifier which has been provided on the downstream stage of a prior art VOA thus far.

[0028] (Second Embodiment)

[0029]FIG. 2 is a plan view showing the second embodiment of the invention wherein there is shown an array made up of four channels, of which each is formed of the variable optical gain control device as described in connection with the first embodiment with reference to FIG. 1. Each of input ports 201, 205, 209, and 213 as the first port group is connected with an optical fiber each, to which an optical signal having a wavelength band of 1.55 μm is inputted. A pump laser light having a wavelength of 1.48 μm or 0.98 μm is inputted from each of input ports 203, 207, 211, and 215 as the second port group.

[0030] Each of optical signals inputted from input ports 201, 205, 209, and 213 is combined with each of pump laser lights by means of each directional coupler of the first directional coupler portion group, respectively. Each of combined optical signals is divided into two optical signals by means of each of directional couplers 221 located on the left side of the Mach-Zehnder portion. These two divided optical signals are given to two arms respectively such that the ratio of their luminous intensities becomes 1:1. When the temperature of the heater 223 in each channel is made to change, the refractive index of each optical waveguide path located immediately below the heater is changed due to the thermo-optic effect, thus the phase of the optical signal propagating through the arm immediately below the heater being changed. Then, the phase-changed wave and the phase-unchanged wave are combined by each directional coupler 222 and the luminous intensity of the optical signal in each channel is controlled according to the phase difference between two waves.

[0031] Each of controlled optical signals is separated from the pump laser light by the directional coupler of the second directional coupler portion group. As the optical waveguide path is doped with erbium, the amplified optical signal is outputted from the output ports 202, 206, 210, and 214 as the third port group.

[0032] As described in the above, according to the second embodiment of the invention, a plurality of (for instance four) variable optical gain control devices as explained in the first embodiment can be arranged to form an array made up of a plurality of (for instance four) channels. This variable optical gain control device is able to control the luminous intensity of the optical signal while the optical signal is amplified, so that simultaneous gain control of a plurality of optical signal waves becomes possible in the WDM communication. Furthermore, it becomes possible to design the device having a more compact size.

[0033] (Third Embodiment)

[0034]FIG. 3 is a plan view showing the third embodiment of the invention. In the first and second embodiments, erbium is doped over the entirety of the optical waveguide path, so that the entire optical circuit acts as an optical amplification circuit. In the third embodiment, however, erbium is not doped to the Mach-Zehnder portion 302 and only the first and second WDM coupler portions 301 and 303 are doped with Er to form optical amplification portions, respectively. Accordingly, the third embodiment is similar to the first embodiment except the point that no optical amplification takes place in the Mach-Zehnder portion 302. Therefore, it is omitted to describe in detail the structure and operation of the device according to the third embodiment.

[0035] As described in the above, the variable optical gain control device according to the third embodiment is constituted such that the Mach-Zehnder portion is given neither erbium nor the amplification function. Consequently, it becomes possible to individually set parameters with regard to the Mach-Zehnder portion and the optical amplification portion, so that there is increased the degree of freedom in the design of the entire optical circuit.

[0036] In the first through third embodiments of the invention, the luminous intensity control portion is constituted by means of a circuit of the Mach-Zehnder type. However, the invention is not limited to these illustrative examples. It may be possible to use a circuit of the directional coupler type or a circuit of the Y-shaped branch type. In case of using a circuit of the directional coupler type, the luminous intensity is controlled by means of the coupling length or the degree of coupling. On the other hand, in case of using a circuit of the Y-shaped branch type, the luminous intensity is controlled by controlling the form of the optical waveguide path.

[0037] Furthermore, the embodiments of the invention are explained about the case where all the optical waveguide paths constituting the entire optical circuit are made of silicon dioxide as a principal material, but it is not limited to silicon dioxide. It is possible to use in part an optical waveguide path made of a certain organic material in combination with those which are made of silicon dioxide. Still further, in the embodiments, the heater is made of chromium but it may be made of other proper materials. Still further, in the second embodiment, it is described that four channels of the variable optical gain control devices are arrayed on the same flat face of the substrate. However, this is just illustrative to the last and the number of channels is not limited to the number as described in the second embodiment.

[0038] As discussed in the above, according to the invention, there are constituted on the same flat face of the substrate a luminous intensity control portion which is formed by using a circuit of the Mach-Zehnder type and an optical amplification portion which is created by doping the optical waveguide paths with a rare earth element. With this, it becomes possible to amplify the optical signal while the luminous intensity control of the optical signal is executed without any attenuation of the optical signal, which has been caused in the prior.

[0039] While some preferred embodiments of the invention have been shown and described in the above with reference to the accompanying drawings, the invention is not limited to such examples. Needless to say, it will be apparent that those skilled in the art would be able to make various changes and modifications within the category of technical thoughts as recited in the scope of claim for patent attached hereto, and it is understood that those changes and modifications naturally belong to the technical category of the invention. 

What is claimed is:
 1. A variable optical gain control device having an optical circuit constituted by means of optical waveguide paths formed on a flat face of a substrate, said optical circuit comprising: the first port for receiving an optical signal inputted thereto; the second port for receiving an excitation light inputted thereto; the first directional coupler portion doped with a rare earth element, for joining up said optical signal and said excitation light and amplifying said optical signal by the function of said excitation light; a luminous intensity control portion doped with a rare earth element, for controlling the luminous intensity of said optical signal; the second directional coupler portion doped with a rare earth element, for separating said optical signal of which the luminous intensity has been controlled, from said excitation light; the third port for outputting said optical signal of which the luminous intensity has been controlled; and the fourth port for outputting said excitation light.
 2. A variable optical gain control device as claimed in claim 1, wherein said rare earth element is erbium.
 3. A variable optical gain control device as claimed in claim 1, wherein said optical signal is a signal having a wavelength of 1.55 μm band.
 4. A variable optical gain control device as claimed in claim 1, wherein said excitation light is a light having a wavelength of 1.48 μm or 0.98 μm.
 5. A variable optical gain control device as claimed in claim 1, wherein said luminous intensity control portion is constituted with a circuit of the Mach-Zehnder type.
 6. A variable optical gain control device as claimed in claim 5, wherein said Mach-Zehnder type circuit has such a structure that is formed by again joining up two of branch optical waveguide paths branching off a single optical waveguide path and one of said two branch optical waveguide paths is provided with a heater.
 7. A variable optical gain control device having an optical circuit constituted by means of optical waveguide paths formed on a flat face of a substrate, said optical circuit comprising: the first port group for receiving optical signals inputted thereto in response to a plurality of different optical signal wavelengths; the second port group for receiving excitation lights inputted thereto in response to a plurality of said different optical signal wavelengths; the first directional coupler portion group doped with a rare earth element, for joining up said optical signals and said excitation lights, respectively, and amplifying said optical signal by the function of said excitation light; a luminous intensity control portion group doped with a rare earth element, for controlling the luminous intensity of said optical signal; the second directional coupler portion group doped with a rare earth element, for separating said optical signals of which the luminous intensity has been controlled, from said excitation lights; the third port group for outputting each of said optical signals of which the luminous intensity has been controlled; and the fourth port group for outputting each of said excitation lights.
 8. A variable optical gain control device as claimed in claim 7, wherein said rare earth element is erbium.
 9. A variable optical gain control device as claimed in claim 7, wherein said optical signal is a signal having a wavelength of 1.55 μm band.
 10. A variable optical gain control device as claimed in claim 7, wherein said excitation light is a light having a wavelength of 1.48 μm or 0.98 μm
 11. A variable optical gain control device as claimed in claim 7, wherein said luminous intensity control portion group is constituted with a circuit of the Mach-Zehnder type.
 12. A variable optical gain control device as claimed in claim 11, wherein said Mach-Zehnder type circuit has such a structure that is formed by again joining up two of branch optical waveguide paths branching off a single optical waveguide path and one of said two branch optical waveguide paths is provided with a heater.
 13. A variable optical gain control device having an optical circuit constituted by means of optical waveguide paths formed on a flat face of a substrate, said optical circuit comprising: the first port for receiving an optical signal inputted thereto; the second port for receiving an excitation light inputted thereto; the first directional coupler portion doped with a rare earth element, for joining up said optical signal and said excitation light and amplifying said optical signal by the function of said excitation light; a luminous intensity control portion for controlling the luminous intensity of said optical signal; the second directional coupler portion doped with a rare earth element, for separating said optical signal of which the luminous intensity has been controlled, from said excitation light; the third port for outputting said optical signal of which the luminous intensity has been controlled; and the fourth port for outputting said excitation light.
 14. A variable optical gain control device as claimed in claim 13, wherein said rare earth element is erbium.
 15. A variable optical gain control device as claimed in claim 13, wherein said optical signal is a signal having a wavelength of 1.55 μm band.
 16. A variable optical gain control device as claimed in claim 13, wherein said excitation signal is a light having a wavelength of 1.48 μm or 0.98 μm.
 17. A variable optical gain control device as claimed in claim 13, wherein said luminous intensity control portion is constituted with a circuit of the Mach-Zehnder type.
 18. A variable optical gain control device as claimed in claim 17, wherein said Mach-Zehnder type circuit has such a structure that is formed by again joining up two of branch optical waveguide paths branching off a single optical waveguide path and one of said two branch optical waveguide paths is provided with a heater. 